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        <h1 id="1-异步"><a href="#1-异步" class="headerlink" title="1. 异步"></a>1. 异步</h1><h2 id="1-1-什么是单线程，和异步有什么关系"><a href="#1-1-什么是单线程，和异步有什么关系" class="headerlink" title="1.1. 什么是单线程，和异步有什么关系"></a>1.1. 什么是单线程，和异步有什么关系</h2><h3 id="1-1-1-单线程：只有一个线程，同一时间只能做一件事情"><a href="#1-1-1-单线程：只有一个线程，同一时间只能做一件事情" class="headerlink" title="1.1.1. 单线程：只有一个线程，同一时间只能做一件事情"></a>1.1.1. 单线程：只有一个线程，同一时间只能做一件事情</h3><figure class="highlight javascript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">// 循环运行期间，JS执行和DOM渲染暂时卡顿</span></span><br><span class="line"><span class="built_in">console</span>.log(<span class="string">'start'</span>);</span><br><span class="line"><span class="keyword">var</span> i, sum = <span class="number">0</span>;</span><br><span class="line"><span class="keyword">for</span> (i = <span class="number">0</span>; i &lt; <span class="number">1000000000</span>; i++)&#123;</span><br><span class="line">  sum += i;</span><br><span class="line">&#125;</span><br><span class="line"><span class="built_in">console</span>.log(sum); <span class="comment">// 上面循环很长,导致打印卡顿</span></span><br><span class="line"></span><br><span class="line"><span class="comment">// alert不处理，JS执行和DOM渲染暂时卡顿</span></span><br><span class="line"><span class="built_in">console</span>.log(<span class="number">1</span>);</span><br><span class="line">alert(<span class="string">'hello'</span>) <span class="comment">// 弹框出来,不点确认的话下面的2是打印不出来的</span></span><br><span class="line"><span class="built_in">console</span>.log(<span class="number">2</span>);</span><br></pre></td></tr></table></figure>
<h3 id="1-1-2-原因：避免-DOM-渲染的冲突"><a href="#1-1-2-原因：避免-DOM-渲染的冲突" class="headerlink" title="1.1.2. 原因：避免 DOM 渲染的冲突"></a>1.1.2. 原因：避免 DOM 渲染的冲突</h3><ul>
<li>浏览器需要渲染 DOM</li>
<li>JS 可以修改 DOM 结构</li>
<li>JS 执行的时候，浏览器 DOM 渲染就会停止</li>
<li>两段 JS 也不能同时执行（同时修改 DOM 就冲突了）</li>
<li>webworker 支持多线程，但是不能访问 DOM</li>
</ul>
<h3 id="1-1-3-解决方案：异步"><a href="#1-1-3-解决方案：异步" class="headerlink" title="1.1.3. 解决方案：异步"></a>1.1.3. 解决方案：异步</h3><p>异步是单线程的唯一解决方案</p>
<figure class="highlight javascript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br></pre></td><td class="code"><pre><span class="line"><span class="built_in">console</span>.log(<span class="number">100</span>);</span><br><span class="line">setTimeout(<span class="function"><span class="params">()</span> =&gt;</span> &#123;</span><br><span class="line">  <span class="built_in">console</span>.log(<span class="number">200</span>);</span><br><span class="line">&#125;, <span class="number">1000</span>);</span><br><span class="line"><span class="built_in">console</span>.log(<span class="number">300</span>);</span><br><span class="line"><span class="built_in">console</span>.log(<span class="number">400</span>);</span><br><span class="line"></span><br><span class="line"><span class="comment">// 100</span></span><br><span class="line"><span class="comment">// 300</span></span><br><span class="line"><span class="comment">// 400</span></span><br><span class="line"><span class="comment">// 200</span></span><br></pre></td></tr></table></figure>
<figure class="highlight javascript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br></pre></td><td class="code"><pre><span class="line"><span class="built_in">console</span>.log(<span class="number">100</span>);</span><br><span class="line">$.ajax(&#123;</span><br><span class="line">  url: <span class="string">'xxx'</span>,</span><br><span class="line">  success: <span class="function"><span class="keyword">function</span>(<span class="params">result</span>) </span>&#123;</span><br><span class="line">    <span class="built_in">console</span>.log(result);</span><br><span class="line">  &#125;</span><br><span class="line">&#125;)</span><br><span class="line"><span class="built_in">console</span>.log(<span class="number">300</span>);</span><br><span class="line"><span class="built_in">console</span>.log(<span class="number">400</span>);</span><br></pre></td></tr></table></figure>
<ul>
<li>问题一：没按照书写方式执行，可读性差</li>
<li>问题二：callback 中不容易模块化</li>
</ul>
<h3 id="1-1-4-问题解答"><a href="#1-1-4-问题解答" class="headerlink" title="1.1.4. 问题解答"></a>1.1.4. 问题解答</h3><ul>
<li>单线程就是同一时间只能做一件事，两段 JS 不能同时执行</li>
<li>原因就是为了避免 DOM 渲染的冲突</li>
<li>异步算是一种“无奈”的解决方案，虽然很高效，但是还是有很多问题存在</li>
</ul>
<h2 id="1-2-什么是-event-loop-事件轮询"><a href="#1-2-什么是-event-loop-事件轮询" class="headerlink" title="1.2. 什么是 event-loop 事件轮询"></a>1.2. 什么是 event-loop 事件轮询</h2><ul>
<li>事件轮询：JS 实现异步的具体解决方案<ul>
<li>同步代码，直接执行</li>
<li>异步函数先放在异步队列中</li>
<li>待同步函数执行完毕，轮询执行异步队列的函数</li>
</ul>
</li>
</ul>
<h2 id="1-3-是否用过-jQuery-的-Deferred"><a href="#1-3-是否用过-jQuery-的-Deferred" class="headerlink" title="1.3. 是否用过 jQuery 的 Deferred"></a>1.3. 是否用过 jQuery 的 Deferred</h2><h3 id="1-3-1-jQuery-1-5-的变化"><a href="#1-3-1-jQuery-1-5-的变化" class="headerlink" title="1.3.1. jQuery 1.5 的变化"></a>1.3.1. jQuery 1.5 的变化</h3><p>jQuery1.5 之前</p>
<figure class="highlight javascript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">var</span> ajax = $.ajax(&#123;</span><br><span class="line">  url: <span class="string">'data.json'</span>,</span><br><span class="line">  success: <span class="function"><span class="keyword">function</span> (<span class="params"></span>) </span>&#123;</span><br><span class="line">    <span class="built_in">console</span>.log(<span class="string">'success1'</span>);</span><br><span class="line">    <span class="built_in">console</span>.log(<span class="string">'success2'</span>);</span><br><span class="line">    <span class="built_in">console</span>.log(<span class="string">'success3'</span>);</span><br><span class="line">  &#125;,</span><br><span class="line">  error: <span class="function"><span class="keyword">function</span> (<span class="params"></span>) </span>&#123;</span><br><span class="line">    <span class="built_in">console</span>.log(<span class="string">'error'</span>);</span><br><span class="line">  &#125;</span><br><span class="line">&#125;)</span><br><span class="line"><span class="built_in">console</span>.log(ajax); <span class="comment">// 返回一个 XHR 对象</span></span><br></pre></td></tr></table></figure>
<p>jQuery1.5 之后</p>
<figure class="highlight javascript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">var</span> ajax = $.ajax(<span class="string">'data.json'</span>)</span><br><span class="line">ajax.done(<span class="function"><span class="keyword">function</span> (<span class="params"></span>) </span>&#123;</span><br><span class="line">    <span class="built_in">console</span>.log(<span class="string">'success1'</span>);</span><br><span class="line">  &#125;)</span><br><span class="line">  .fail(<span class="function"><span class="keyword">function</span> (<span class="params"></span>) </span>&#123;</span><br><span class="line">    <span class="built_in">console</span>.log(<span class="string">'error'</span>);</span><br><span class="line">  &#125;)</span><br><span class="line">  .done(<span class="function"><span class="keyword">function</span> (<span class="params"></span>) </span>&#123;</span><br><span class="line">    <span class="built_in">console</span>.log(<span class="string">'success2'</span>);</span><br><span class="line">  &#125;)</span><br><span class="line"><span class="built_in">console</span>.log(ajax); <span class="comment">// 返回一个deferred 对象</span></span><br></pre></td></tr></table></figure>
<figure class="highlight javascript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">// 很像 Promise 的写法</span></span><br><span class="line"><span class="keyword">var</span> ajax = $.ajax(<span class="string">'data.json'</span>)</span><br><span class="line">ajax.then(<span class="function"><span class="keyword">function</span> (<span class="params"></span>) </span>&#123;</span><br><span class="line">    <span class="built_in">console</span>.log(<span class="string">'success1'</span>)</span><br><span class="line">  &#125;, <span class="function"><span class="keyword">function</span> (<span class="params"></span>) </span>&#123;</span><br><span class="line">    <span class="built_in">console</span>.log(<span class="string">'error1'</span>)</span><br><span class="line">  &#125;)</span><br><span class="line">  .then(<span class="function"><span class="keyword">function</span> (<span class="params"></span>) </span>&#123;</span><br><span class="line">    <span class="built_in">console</span>.log(<span class="string">'success2'</span>)</span><br><span class="line">  &#125;, <span class="function"><span class="keyword">function</span> (<span class="params"></span>) </span>&#123;</span><br><span class="line">    <span class="built_in">console</span>.log(<span class="string">'error2'</span>)</span><br><span class="line">  &#125;)</span><br></pre></td></tr></table></figure>
<ul>
<li>无法改变 JS 异步和单线程的本质</li>
<li>只能从写法上杜绝 callback 这种形式</li>
<li>它是一种语法糖形式，但是解耦了代码</li>
<li>很好的体现了开放封闭原则（对扩展开放，对修改封闭）</li>
</ul>
<h3 id="1-3-2-使用-jQuery-Deferred"><a href="#1-3-2-使用-jQuery-Deferred" class="headerlink" title="1.3.2. 使用 jQuery Deferred"></a>1.3.2. 使用 jQuery Deferred</h3><p>给出一段非常简单的异步操作代码，使用 setTimeout 函数</p>
<figure class="highlight javascript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">var</span> wait = <span class="function"><span class="keyword">function</span> (<span class="params"></span>) </span>&#123;</span><br><span class="line">  <span class="keyword">var</span> task = <span class="function"><span class="keyword">function</span> (<span class="params"></span>) </span>&#123;</span><br><span class="line">    <span class="built_in">console</span>.log(<span class="string">'执行完成'</span>);</span><br><span class="line">  &#125;</span><br><span class="line">  setTimeout(task, <span class="number">2000</span>)</span><br><span class="line">&#125;</span><br><span class="line">wait()</span><br></pre></td></tr></table></figure>
<p>新增需求：要在执行完成之后进行一些特别复杂的操作，代码可能会很多，而且分好几个步骤</p>
<figure class="highlight javascript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br></pre></td><td class="code"><pre><span class="line"><span class="function"><span class="keyword">function</span> <span class="title">waitHandle</span>(<span class="params"></span>) </span>&#123;</span><br><span class="line">  <span class="keyword">var</span> dtd = $.Deferred()  <span class="comment">// 创建一个deferred对象</span></span><br><span class="line"></span><br><span class="line">  <span class="keyword">var</span> wait = <span class="function"><span class="keyword">function</span> (<span class="params">dtd</span>) </span>&#123; <span class="comment">// 要求传入一个deferred对象</span></span><br><span class="line">    <span class="keyword">var</span> task = <span class="function"><span class="keyword">function</span> (<span class="params"></span>) </span>&#123;</span><br><span class="line">      <span class="built_in">console</span>.log(<span class="string">'执行完成'</span>)</span><br><span class="line">      dtd.resolve() <span class="comment">// 表示异步任务已经完成</span></span><br><span class="line">      dtd.reject() <span class="comment">// 失败</span></span><br><span class="line">    &#125;</span><br><span class="line">    setTimeout(task, <span class="number">2000</span>)</span><br><span class="line">    <span class="keyword">return</span> dtd  <span class="comment">// 要求返回deferred对象</span></span><br><span class="line">  &#125;</span><br><span class="line"></span><br><span class="line">  <span class="comment">// 注意，这里一定要有返回值</span></span><br><span class="line">  <span class="keyword">return</span> wait(dtd)</span><br><span class="line">&#125;</span><br><span class="line"></span><br><span class="line"><span class="keyword">var</span> w = waitHandle()</span><br><span class="line">w.then(<span class="function"><span class="keyword">function</span> (<span class="params"></span>) </span>&#123;</span><br><span class="line">  <span class="built_in">console</span>.log(<span class="string">'ok 1'</span>)</span><br><span class="line">&#125;, <span class="function"><span class="keyword">function</span> (<span class="params"></span>) </span>&#123;</span><br><span class="line">  <span class="built_in">console</span>.log(<span class="string">'err 1'</span>)</span><br><span class="line">&#125;).then(<span class="function"><span class="keyword">function</span> (<span class="params"></span>) </span>&#123;</span><br><span class="line">  <span class="built_in">console</span>.log(<span class="string">'ok 2'</span>)</span><br><span class="line">&#125;, <span class="function"><span class="keyword">function</span> (<span class="params"></span>) </span>&#123;</span><br><span class="line">  <span class="built_in">console</span>.log(<span class="string">'err 2'</span>)</span><br><span class="line">&#125;)</span><br><span class="line"></span><br><span class="line"><span class="comment">// 执行dtd.resolve()则打印ok1 ok2</span></span><br><span class="line"><span class="comment">// 执行dtd.reject()则打印err1 err2</span></span><br></pre></td></tr></table></figure>
<ul>
<li>总结：dtd 的 API 可分成两类，用意不同<ul>
<li>第一类：dtd.resolve dtd.reject</li>
<li>第二类：dtd.then dtd.done dtd.fail</li>
</ul>
</li>
<li>这两类应该分开，否则后果很严重</li>
<li>可以在上面代码最后执行 dtd.reject()试一下后果</li>
</ul>
<h3 id="1-3-3-问题解答"><a href="#1-3-3-问题解答" class="headerlink" title="1.3.3. 问题解答"></a>1.3.3. 问题解答</h3><ul>
<li>可以 jQuery1.5 对 ajax 的改变来举例</li>
<li>说明如何简单的封装、使用 Deferred</li>
<li>说明 promise 和 Deferred 的区别</li>
</ul>
<h2 id="1-4-Promise-的基本使用和原理"><a href="#1-4-Promise-的基本使用和原理" class="headerlink" title="1.4. Promise 的基本使用和原理"></a>1.4. Promise 的基本使用和原理</h2><h3 id="1-4-1-基本语法回顾"><a href="#1-4-1-基本语法回顾" class="headerlink" title="1.4.1. 基本语法回顾"></a>1.4.1. 基本语法回顾</h3><figure class="highlight javascript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br></pre></td><td class="code"><pre><span class="line"><span class="function"><span class="keyword">function</span> <span class="title">loadImg</span>(<span class="params">src</span>) </span>&#123;</span><br><span class="line">    <span class="keyword">var</span> promise = <span class="keyword">new</span> <span class="built_in">Promise</span>(<span class="function"><span class="keyword">function</span> (<span class="params">resolve, reject</span>) </span>&#123;</span><br><span class="line">        <span class="keyword">var</span> img = <span class="built_in">document</span>.createElement(<span class="string">'img'</span>)</span><br><span class="line">        <span class="comment">// throw new Error('自定义错误')</span></span><br><span class="line">        img.onload = <span class="function"><span class="keyword">function</span> (<span class="params"></span>) </span>&#123;</span><br><span class="line">            resolve(img)</span><br><span class="line">        &#125;</span><br><span class="line">        img.onerror = <span class="function"><span class="keyword">function</span> (<span class="params"></span>) </span>&#123;</span><br><span class="line">            reject(<span class="string">'图片加载失败'</span>)</span><br><span class="line">        &#125;</span><br><span class="line">        img.src = src</span><br><span class="line">    &#125;)</span><br><span class="line">    <span class="keyword">return</span> promise</span><br><span class="line">&#125;</span><br><span class="line"></span><br><span class="line"><span class="keyword">var</span> src = <span class="string">'https://www.imooc.com/static/img/index/logo_new.png'</span></span><br><span class="line"><span class="keyword">var</span> result = loadImg(src)</span><br><span class="line">result.then(<span class="function"><span class="keyword">function</span> (<span class="params">img</span>) </span>&#123;</span><br><span class="line">  <span class="built_in">console</span>.log(<span class="number">1</span>,img.width)</span><br><span class="line">  <span class="keyword">return</span> img</span><br><span class="line">&#125;, <span class="function"><span class="keyword">function</span> (<span class="params"></span>) </span>&#123;</span><br><span class="line">  <span class="built_in">console</span>.log(<span class="string">'error 1'</span>)</span><br><span class="line">&#125;).then(<span class="function"><span class="keyword">function</span> (<span class="params">img</span>) </span>&#123;</span><br><span class="line">  <span class="built_in">console</span>.log(<span class="number">1</span>, img.height)</span><br><span class="line">&#125;)</span><br></pre></td></tr></table></figure>
<h3 id="1-4-2-异常捕获"><a href="#1-4-2-异常捕获" class="headerlink" title="1.4.2. 异常捕获"></a>1.4.2. 异常捕获</h3><p>then 只接受一个参数，最后统一用 catch 捕获异常</p>
<figure class="highlight javascript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">var</span> src = <span class="string">'https://www.imooc.com/static/img/index/logo_new.png'</span></span><br><span class="line"><span class="keyword">var</span> result = loadImg(src)</span><br><span class="line">result.then(<span class="function"><span class="keyword">function</span> (<span class="params">img</span>) </span>&#123;</span><br><span class="line">  <span class="built_in">console</span>.log(<span class="number">1</span>, img.width)</span><br><span class="line">  <span class="keyword">return</span> img</span><br><span class="line">&#125;).then(<span class="function"><span class="keyword">function</span> (<span class="params">img</span>) </span>&#123;</span><br><span class="line">  <span class="built_in">console</span>.log(<span class="number">2</span>, img.height)</span><br><span class="line">&#125;).catch(<span class="function"><span class="keyword">function</span> (<span class="params">ex</span>) </span>&#123;</span><br><span class="line">  <span class="comment">// 统一捕获异常</span></span><br><span class="line">  <span class="built_in">console</span>.log(ex)</span><br><span class="line">&#125;)</span><br></pre></td></tr></table></figure>
<h3 id="1-4-3-多个串联"><a href="#1-4-3-多个串联" class="headerlink" title="1.4.3. 多个串联"></a>1.4.3. 多个串联</h3><figure class="highlight javascript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">var</span> src1 = <span class="string">'https://www.imooc.com/static/img/index/logo_new.png'</span></span><br><span class="line"><span class="keyword">var</span> result1 = loadImg(src1)</span><br><span class="line"><span class="keyword">var</span> src2 = <span class="string">'https://img1.mukewang.com/545862fe00017c2602200220-100-100.jpg'</span></span><br><span class="line"><span class="keyword">var</span> result2 = loadImg(src2)</span><br><span class="line"></span><br><span class="line"><span class="comment">// 链式操作</span></span><br><span class="line">result1.then(<span class="function"><span class="keyword">function</span> (<span class="params">img1</span>) </span>&#123;</span><br><span class="line">    <span class="built_in">console</span>.log(<span class="string">'第一个图片加载完成'</span>, img1.width)</span><br><span class="line">    <span class="keyword">return</span> result2  <span class="comment">// 重要！！！</span></span><br><span class="line">&#125;).then(<span class="function"><span class="keyword">function</span> (<span class="params">img2</span>) </span>&#123;</span><br><span class="line">    <span class="built_in">console</span>.log(<span class="string">'第二个图片加载完成'</span>, img2.width)</span><br><span class="line">&#125;).catch(<span class="function"><span class="keyword">function</span> (<span class="params">ex</span>) </span>&#123;</span><br><span class="line">  <span class="comment">// 最后统一catch</span></span><br><span class="line">    <span class="built_in">console</span>.log(ex)</span><br><span class="line">&#125;)</span><br></pre></td></tr></table></figure>
<h3 id="1-4-4-Promise-all-和-Promise-race"><a href="#1-4-4-Promise-all-和-Promise-race" class="headerlink" title="1.4.4. Promise.all 和 Promise.race"></a>1.4.4. Promise.all 和 Promise.race</h3><figure class="highlight javascript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">// Promise.all 接收一个 promise 对象的数组</span></span><br><span class="line"><span class="comment">// 待全部完成之后，统一执行 success</span></span><br><span class="line"><span class="built_in">Promise</span>.all([result1, result2]).then(<span class="function">(<span class="params">datas</span>) =&gt;</span> &#123;</span><br><span class="line">  <span class="comment">// 接收到的datas是一个数组，依次包含了多个promise返回的内容</span></span><br><span class="line">  <span class="built_in">console</span>.log(datas[<span class="number">0</span>])</span><br><span class="line">  <span class="built_in">console</span>.log(datas[<span class="number">1</span>])</span><br><span class="line">&#125;)</span><br><span class="line"></span><br><span class="line"><span class="comment">// Promise.race 接收一个包含多个promise对象的数组</span></span><br><span class="line"><span class="comment">// 只要有一个完成,就执行seccess</span></span><br><span class="line"><span class="built_in">Promise</span>.race([result1, result2]).then(<span class="function">(<span class="params">data</span>) =&gt;</span> &#123;</span><br><span class="line">  <span class="comment">// data即最先执行完成的promise的返回值</span></span><br><span class="line">  <span class="built_in">console</span>.log(data)</span><br><span class="line">&#125;)</span><br></pre></td></tr></table></figure>
<h3 id="1-4-5-Promise-标准"><a href="#1-4-5-Promise-标准" class="headerlink" title="1.4.5. Promise 标准"></a>1.4.5. Promise 标准</h3><ul>
<li>“标准”<ul>
<li>任何技术推广使用都需要一套标准来支撑</li>
<li>如 html js css http 等，无规矩不方圆</li>
<li>任何不符合标准的东西，终将会被用户抛弃</li>
<li>不要挑战标准，不要自造标准</li>
</ul>
</li>
<li>状态变化<ul>
<li>三种状态：pending、fulfilled、rejected<ul>
<li>初始状态：pending</li>
<li>pending 变为 fulfilled，或者 pending 变为 rejected</li>
<li>状态变化不可逆</li>
</ul>
</li>
</ul>
</li>
<li>then<ul>
<li>Promise 实例必须实现 then 这个方法</li>
<li>then()必须可以接收两个函数作为参数</li>
<li>then()返回的必须是个 Promise 实例</li>
</ul>
</li>
</ul>
<h3 id="1-4-6-问题解答"><a href="#1-4-6-问题解答" class="headerlink" title="1.4.6. 问题解答"></a>1.4.6. 问题解答</h3><ul>
<li>基础语法</li>
<li>如何捕获异常（Error 和 reject 都要考虑）</li>
<li>多个串联，链式执行的好处</li>
<li>Promise.all 和 Promise.race</li>
<li>Promise 标准</li>
</ul>
<h2 id="1-5-介绍一下-async-await-（和-Promise-的区别、联系）"><a href="#1-5-介绍一下-async-await-（和-Promise-的区别、联系）" class="headerlink" title="1.5. 介绍一下 async/await （和 Promise 的区别、联系）"></a>1.5. 介绍一下 async/await （和 Promise 的区别、联系）</h2><ul>
<li>then 只是将 callback 拆分了</li>
<li>async/await 是最直接的同步写法</li>
</ul>
<p>用法：</p>
<ul>
<li>使用 await，函数必须用 async 标识</li>
<li>await 后面跟的是一个 Promise 实例</li>
<li>需要 babel-polyfill</li>
</ul>
<figure class="highlight javascript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">const</span> load = <span class="keyword">async</span> <span class="function"><span class="keyword">function</span> (<span class="params"></span>) </span>&#123;</span><br><span class="line">  <span class="keyword">const</span> result1 = <span class="keyword">await</span> loadImg(src1)</span><br><span class="line">  <span class="built_in">console</span>.log(result1)</span><br><span class="line">  <span class="keyword">const</span> result2 = <span class="keyword">await</span> loadImg(src2)</span><br><span class="line">  <span class="built_in">console</span>.log(result2)</span><br><span class="line">&#125;</span><br><span class="line"></span><br><span class="line">load()</span><br></pre></td></tr></table></figure>
<h3 id="1-5-1-问题解答"><a href="#1-5-1-问题解答" class="headerlink" title="1.5.1. 问题解答"></a>1.5.1. 问题解答</h3><ul>
<li>基本语法</li>
<li>async/await 使用了 Promise，并没有和 Promise 冲突<ul>
<li>promise 是对异步回调的一个封装，promise 标准中，如果封装完了之后要使用的话要有一个.then .then .catch 的写法</li>
<li>async/await 使用了 promise 的封装，但是是完全同步的写法。并不是取代了 promise，而是和 promise 进行了完美的兼容</li>
</ul>
</li>
<li>但是，任何写法的改变都改变不了 JS 单线程、异步的本质</li>
</ul>
<h2 id="1-6-总结一下当前-JS-解决异步的方案"><a href="#1-6-总结一下当前-JS-解决异步的方案" class="headerlink" title="1.6. 总结一下当前 JS 解决异步的方案"></a>1.6. 总结一下当前 JS 解决异步的方案</h2><ul>
<li>jQuery Deferred</li>
<li>Promise</li>
<li>Async/Await</li>
<li>Generator（不推荐）<ul>
<li>原理比较复杂</li>
<li>不是异步的直接替代方式</li>
<li>有更好更简洁的解决方案 async/await</li>
<li>koa 也弃用了</li>
</ul>
</li>
</ul>

      
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              <div class="post-toc-content"><ol class="nav"><li class="nav-item nav-level-1"><a class="nav-link" href="#1-异步"><span class="nav-text">1. 异步</span></a><ol class="nav-child"><li class="nav-item nav-level-2"><a class="nav-link" href="#1-1-什么是单线程，和异步有什么关系"><span class="nav-text">1.1. 什么是单线程，和异步有什么关系</span></a><ol class="nav-child"><li class="nav-item nav-level-3"><a class="nav-link" href="#1-1-1-单线程：只有一个线程，同一时间只能做一件事情"><span class="nav-text">1.1.1. 单线程：只有一个线程，同一时间只能做一件事情</span></a></li><li class="nav-item nav-level-3"><a class="nav-link" href="#1-1-2-原因：避免-DOM-渲染的冲突"><span class="nav-text">1.1.2. 原因：避免 DOM 渲染的冲突</span></a></li><li class="nav-item nav-level-3"><a class="nav-link" href="#1-1-3-解决方案：异步"><span class="nav-text">1.1.3. 解决方案：异步</span></a></li><li class="nav-item nav-level-3"><a class="nav-link" href="#1-1-4-问题解答"><span class="nav-text">1.1.4. 问题解答</span></a></li></ol></li><li class="nav-item nav-level-2"><a class="nav-link" href="#1-2-什么是-event-loop-事件轮询"><span class="nav-text">1.2. 什么是 event-loop 事件轮询</span></a></li><li class="nav-item nav-level-2"><a class="nav-link" href="#1-3-是否用过-jQuery-的-Deferred"><span class="nav-text">1.3. 是否用过 jQuery 的 Deferred</span></a><ol class="nav-child"><li class="nav-item nav-level-3"><a class="nav-link" href="#1-3-1-jQuery-1-5-的变化"><span class="nav-text">1.3.1. jQuery 1.5 的变化</span></a></li><li class="nav-item nav-level-3"><a class="nav-link" href="#1-3-2-使用-jQuery-Deferred"><span class="nav-text">1.3.2. 使用 jQuery Deferred</span></a></li><li class="nav-item nav-level-3"><a class="nav-link" href="#1-3-3-问题解答"><span class="nav-text">1.3.3. 问题解答</span></a></li></ol></li><li class="nav-item nav-level-2"><a class="nav-link" href="#1-4-Promise-的基本使用和原理"><span class="nav-text">1.4. Promise 的基本使用和原理</span></a><ol class="nav-child"><li class="nav-item nav-level-3"><a class="nav-link" href="#1-4-1-基本语法回顾"><span class="nav-text">1.4.1. 基本语法回顾</span></a></li><li class="nav-item nav-level-3"><a class="nav-link" href="#1-4-2-异常捕获"><span class="nav-text">1.4.2. 异常捕获</span></a></li><li class="nav-item nav-level-3"><a class="nav-link" href="#1-4-3-多个串联"><span class="nav-text">1.4.3. 多个串联</span></a></li><li class="nav-item nav-level-3"><a class="nav-link" href="#1-4-4-Promise-all-和-Promise-race"><span class="nav-text">1.4.4. Promise.all 和 Promise.race</span></a></li><li class="nav-item nav-level-3"><a class="nav-link" href="#1-4-5-Promise-标准"><span class="nav-text">1.4.5. Promise 标准</span></a></li><li class="nav-item nav-level-3"><a class="nav-link" href="#1-4-6-问题解答"><span class="nav-text">1.4.6. 问题解答</span></a></li></ol></li><li class="nav-item nav-level-2"><a class="nav-link" href="#1-5-介绍一下-async-await-（和-Promise-的区别、联系）"><span class="nav-text">1.5. 介绍一下 async/await （和 Promise 的区别、联系）</span></a><ol class="nav-child"><li class="nav-item nav-level-3"><a class="nav-link" href="#1-5-1-问题解答"><span class="nav-text">1.5.1. 问题解答</span></a></li></ol></li><li class="nav-item nav-level-2"><a class="nav-link" href="#1-6-总结一下当前-JS-解决异步的方案"><span class="nav-text">1.6. 总结一下当前 JS 解决异步的方案</span></a></li></ol></li></ol></div>
            

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                    if(start < 0){
                      start = 0;
                    }
                    if (end < position + word.length) {
                      end = position + word.length;
                    }
                    if(end > content.length){
                      end = content.length;
                    }
                    slicesOfContent.push(mergeIntoSlice(content, start, end, indexOfContent));
                  }

                  // sort slices in content by search text's count and hits' count

                  slicesOfContent.sort(function (sliceLeft, sliceRight) {
                    if (sliceLeft.searchTextCount !== sliceRight.searchTextCount) {
                      return sliceRight.searchTextCount - sliceLeft.searchTextCount;
                    } else if (sliceLeft.hits.length !== sliceRight.hits.length) {
                      return sliceRight.hits.length - sliceLeft.hits.length;
                    } else {
                      return sliceLeft.start - sliceRight.start;
                    }
                  });

                  // select top N slices in content

                  var upperBound = parseInt('1');
                  if (upperBound >= 0) {
                    slicesOfContent = slicesOfContent.slice(0, upperBound);
                  }

                  // highlight title and content

                  function highlightKeyword(text, slice) {
                    var result = '';
                    var prevEnd = slice.start;
                    slice.hits.forEach(function (hit) {
                      result += text.substring(prevEnd, hit.position);
                      var end = hit.position + hit.length;
                      result += '<b class="search-keyword">' + text.substring(hit.position, end) + '</b>';
                      prevEnd = end;
                    });
                    result += text.substring(prevEnd, slice.end);
                    return result;
                  }

                  var resultItem = '';

                  if (slicesOfTitle.length != 0) {
                    resultItem += "<li><a href='" + articleUrl + "' class='search-result-title'>" + highlightKeyword(title, slicesOfTitle[0]) + "</a>";
                  } else {
                    resultItem += "<li><a href='" + articleUrl + "' class='search-result-title'>" + title + "</a>";
                  }

                  slicesOfContent.forEach(function (slice) {
                    resultItem += "<a href='" + articleUrl + "'>" +
                      "<p class=\"search-result\">" + highlightKeyword(content, slice) +
                      "...</p>" + "</a>";
                  });

                  resultItem += "</li>";
                  resultItems.push({
                    item: resultItem,
                    searchTextCount: searchTextCount,
                    hitCount: hitCount,
                    id: resultItems.length
                  });
                }
              })
            };
            if (keywords.length === 1 && keywords[0] === "") {
              resultContent.innerHTML = '<div id="no-result"><i class="fa fa-search fa-5x" /></div>'
            } else if (resultItems.length === 0) {
              resultContent.innerHTML = '<div id="no-result"><i class="fa fa-frown-o fa-5x" /></div>'
            } else {
              resultItems.sort(function (resultLeft, resultRight) {
                if (resultLeft.searchTextCount !== resultRight.searchTextCount) {
                  return resultRight.searchTextCount - resultLeft.searchTextCount;
                } else if (resultLeft.hitCount !== resultRight.hitCount) {
                  return resultRight.hitCount - resultLeft.hitCount;
                } else {
                  return resultRight.id - resultLeft.id;
                }
              });
              var searchResultList = '<ul class=\"search-result-list\">';
              resultItems.forEach(function (result) {
                searchResultList += result.item;
              })
              searchResultList += "</ul>";
              resultContent.innerHTML = searchResultList;
            }
          }

          if ('auto' === 'auto') {
            input.addEventListener('input', inputEventFunction);
          } else {
            $('.search-icon').click(inputEventFunction);
            input.addEventListener('keypress', function (event) {
              if (event.keyCode === 13) {
                inputEventFunction();
              }
            });
          }

          // remove loading animation
          $(".local-search-pop-overlay").remove();
          $('body').css('overflow', '');

          proceedsearch();
        }
      });
    }

    // handle and trigger popup window;
    $('.popup-trigger').click(function(e) {
      e.stopPropagation();
      if (isfetched === false) {
        searchFunc(path, 'local-search-input', 'local-search-result');
      } else {
        proceedsearch();
      };
    });

    $('.popup-btn-close').click(onPopupClose);
    $('.popup').click(function(e){
      e.stopPropagation();
    });
    $(document).on('keyup', function (event) {
      var shouldDismissSearchPopup = event.which === 27 &&
        $('.search-popup').is(':visible');
      if (shouldDismissSearchPopup) {
        onPopupClose();
      }
    });
  </script>





  

  

  

  
  
  
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  <script>
    
      pbOptions = {};
      
          pbOptions.iconStyle = "box";
      
          pbOptions.boxForm = "horizontal";
      
          pbOptions.position = "bottomCenter";
      
          pbOptions.networks = "Weibo,Wechat,Douban,QQZone,Twitter,Facebook,GooglePlus,Linkedin,Evernote";
      
      new needShareButton('#needsharebutton-postbottom', pbOptions);
    
    
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